Fix choice of comparison operators for cross-type hashed subplans.

Commit bf6c614a rearranged the lookup of the comparison operators
needed in a hashed subplan, and in so doing, broke the cross-type
case: it caused the original LHS-vs-RHS operator to be used to compare
hash table entries too (which of course are all of the RHS type).
This leads to C functions being passed a Datum that is not of the
type they expect, with the usual hazards of crashes and unauthorized
server memory disclosure.

For the set of hashable cross-type operators present in v11 core
Postgres, this bug is nearly harmless on 64-bit machines, which
may explain why it escaped earlier detection.  But it is a live
security hazard on 32-bit machines; and of course there may be
extensions that add more hashable cross-type operators, which
would increase the risk.

Reported by Andreas Seltenreich.  Back-patch to v11 where the
problem came in.

Security: CVE-2019-10209

src/backend/executor/nodeSubplan.c

@@ -860,6 +860,7 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 					i;
 		TupleDesc	tupDescLeft;
 		TupleDesc	tupDescRight;
+		Oid		   *cross_eq_funcoids;
 		TupleTableSlot *slot;
 		List	   *oplist,
 				   *lefttlist,
@@ -923,6 +924,9 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 		sstate->tab_collations = (Oid *) palloc(ncols * sizeof(Oid));
 		sstate->lhs_hash_funcs = (FmgrInfo *) palloc(ncols * sizeof(FmgrInfo));
 		sstate->cur_eq_funcs = (FmgrInfo *) palloc(ncols * sizeof(FmgrInfo));
+		/* we'll need the cross-type equality fns below, but not in sstate */
+		cross_eq_funcoids = (Oid *) palloc(ncols * sizeof(Oid));
+
 		i = 1;
 		foreach(l, oplist)
 		{
@@ -952,7 +956,7 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 			righttlist = lappend(righttlist, tle);
 
 			/* Lookup the equality function (potentially cross-type) */
-			sstate->tab_eq_funcoids[i - 1] = opexpr->opfuncid;
+			cross_eq_funcoids[i - 1] = opexpr->opfuncid;
 			fmgr_info(opexpr->opfuncid, &sstate->cur_eq_funcs[i - 1]);
 			fmgr_info_set_expr((Node *) opexpr, &sstate->cur_eq_funcs[i - 1]);
 
@@ -961,7 +965,9 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 											   NULL, &rhs_eq_oper))
 				elog(ERROR, "could not find compatible hash operator for operator %u",
 					 opexpr->opno);
-			fmgr_info(get_opcode(rhs_eq_oper), &sstate->tab_eq_funcs[i - 1]);
+			sstate->tab_eq_funcoids[i - 1] = get_opcode(rhs_eq_oper);
+			fmgr_info(sstate->tab_eq_funcoids[i - 1],
+					  &sstate->tab_eq_funcs[i - 1]);
 
 			/* Lookup the associated hash functions */
 			if (!get_op_hash_functions(opexpr->opno,
@@ -1003,16 +1009,15 @@ ExecInitSubPlan(SubPlan *subplan, PlanState *parent)
 
 		/*
 		 * Create comparator for lookups of rows in the table (potentially
-		 * across-type comparison).
+		 * cross-type comparisons).
 		 */
 		sstate->cur_eq_comp = ExecBuildGroupingEqual(tupDescLeft, tupDescRight,
 													 &TTSOpsVirtual, &TTSOpsMinimalTuple,
 													 ncols,
 													 sstate->keyColIdx,
-													 sstate->tab_eq_funcoids,
+													 cross_eq_funcoids,
 													 sstate->tab_collations,
 													 parent);
-
 	}
 
 	return sstate;

src/test/regress/expected/subselect.out

@@ -764,6 +764,30 @@ select * from outer_text where (f1, f2) not in (select * from inner_text);
  b  | 
 (2 rows)
 
+--
+-- Another test case for cross-type hashed subplans: comparison of
+-- inner-side values must be done with appropriate operator
+--
+explain (verbose, costs off)
+select 'foo'::text in (select 'bar'::name union all select 'bar'::name);
+             QUERY PLAN              
+-------------------------------------
+ Result
+   Output: (hashed SubPlan 1)
+   SubPlan 1
+     ->  Append
+           ->  Result
+                 Output: 'bar'::name
+           ->  Result
+                 Output: 'bar'::name
+(8 rows)
+
+select 'foo'::text in (select 'bar'::name union all select 'bar'::name);
+ ?column? 
+----------
+ f
+(1 row)
+
 --
 -- Test case for premature memory release during hashing of subplan output
 --

src/test/regress/sql/subselect.sql

@@ -452,6 +452,16 @@ insert into inner_text values ('a', null);
 
 select * from outer_text where (f1, f2) not in (select * from inner_text);
 
+--
+-- Another test case for cross-type hashed subplans: comparison of
+-- inner-side values must be done with appropriate operator
+--
+
+explain (verbose, costs off)
+select 'foo'::text in (select 'bar'::name union all select 'bar'::name);
+
+select 'foo'::text in (select 'bar'::name union all select 'bar'::name);
+
 --
 -- Test case for premature memory release during hashing of subplan output
 --